The epidemiology of autoimmune diseases is characterized by a significant sex dimorphism, with the majority of disorders being more prevalent in women. In a parallel fashion, the immune system shows sex-dependent differences in number and functions of both its innate and its adaptive arms, with women capable to mount a more vigorous response compared to men. This enhanced reactivity may contribute to the stronger defense against infectious agents and to the reasons for which, on the other hand, women are more prone to develop autoimmune diseases. Several factors have been studied and implied to play a role for such an imbalance, most notably sex chromosomes, sex hormones, and gut microbiota differences between sexes. Experimental studies on rodents demonstrate that sex chromosome abnormalities, alterations of gut microbiota composition, and fluctuations of sex hormone concentrations decrease the susceptibility to autoimmunity in female probes or increase it in the male counterparts. Nevertheless, it would be reductive to consider sex only as a risk factor; based on clinical experience, autoimmune disease onset and course differ between men and women in terms of disease progression and severity. Eventually, research has focused on sex as a determinant of antirheumatic treatment response with promising evidence for a further personalized management of patients with autoimmune diseases.

Autoimmune diseases encompass a spectrum of more than 80 chronic and acute conditions that cumulatively affect nearly 5% of the population in Western countries.[1] These disorders are characterized by the breakdown of immune tolerance and result in an immune response with autoantibody production and autoreactive T-cell generation, damage, and dysfunction of different organs. Despite the numerous clinical differences which manifest a wide variability in terms of targeted tissues, age of onset, and response to immunosuppressive therapies, there is one feature that nearly all autoimmune diseases share is the higher prevalence in women with over 80% of patients being of female sex.[2] It is essential to clarify that semantics discriminate gender from sex, with the latter indicating the biology along with the anatomic reproductive system and secondary sex characteristics, while gender referring to the socially constructed characteristics of women and men – such as norms, roles, and relationships of and between groups of women and men – possibly divergent from biology. In general terms and in the medical literature, however, sex and geneder are used as synonyms. The most striking sex differences in autoimmune diseases are observed in Sjogren's syndrome, systemic lupus erythematosus (SLE), primary biliary cholangitis (PBC), autoimmune thyroid disease (including Grave's and Hashimoto's diseases), and systemic sclerosis (SSc)[3][Table 1]. This leads to the common question of defining the sex-related factors that influence susceptibility to autoimmunity and the question of whether gender influences the natural course of disease as well as response to therapy.[4] The objective of the present study is to provide an overview of the mechanisms and possible explanations of the female predominance in autoimmune diseases.

The presence of autoantibodies is a biochemical hallmark of many autoimmune diseases, such as SLE, rheumatoid arthritis (RA), PBC, and autoimmune thyroid disease. The prevalence of autoantibodies in the general population has not been fully clarified; however, different studies tried to assess their distribution. In particular, serum antinuclear (ANA) and anti-extractable nuclear antigen (ENA) were determined in our Isola II study,[5] revealing a strong and predictable female predominance, even though the female-to-male ratio was lower than the corresponding clinical phenotype, while the prevalence of serum autoantibodies (ANA was detected in 18.1% of subjects at titers ≥1:80) is higher than the prevalence of other diseases in general population. This means that, even though sex-related factors intervene to create a female-to-male imbalance in the prevalence of autoantibodies, autoantibody production alone is not sufficient to explain pathology, as they are also present in sera of patients who do not suffer any of these disorders and will not necessarily develop signs and symptoms of rheumatic disease. Thus, it appears that other genetics and environmental factors are involved in immune tolerance loss, although the clear mechanism remains unknown.

Why Women?

Sex is a biological variable influencing both innate and adaptive immune responses via different mechanisms. Men and women differ in their immunological responses to foreign and self-antigens and show distinctions in innate and adaptive immune responses.[6] In general terms, women have an enhanced antibody production and increased cell-mediated responses following immunization, while men produce a more vigorous immune response to infectious organisms. Furthermore, females show higher CD4+ T-cell counts than males, which contributes to an increased CD4/CD8 ratio, higher levels of plasma IgM, and greater T-helper 1 (Th1) cytokine production.[7]

Not only women and men differ in their normal immune response but also there are differences between them in the prevalence, presentation, and severity of autoimmune diseases. This leads to the notion that increased immune system function causes increased susceptibility to autoimmunity in women.

Genetics and sex chromosomes

Men and women differ in the number of X or Y chromosomes in their karyotypes, with females displaying two X chromosomes (one maternal and one paternal X chromosome) and males having a paternal Y chromosome and a maternal X. Many of the genes on the X chromosome regulate immune function and seem to be important in immune-related diseases, these genes encode for proteins such as pattern recognition receptors-(for example TLR7 and TLR8), cytokine receptors, and transcription factors (like Foxp3).[6] An important role for genes on X chromosome in immune system function is highlighted by the finding that major sex chromosome abnormalities are associated with an altered susceptibility for some autoimmune diseases compared to sex-matched general population,[8] demonstrating the important role played by X chromosome in influencing susceptibility to autoimmunity.[9] In fact, women with Turner syndrome, who have a 45, X0 karyotype or major X chromosome deletions, have lower IgG and IgM levels and lower T-cell and B-cell levels compared to XX females. Interestingly, patients with Turner disease exhibit an increased susceptibility to autoimmune diseases than female controls, in particular for those autoimmune diseases with a male predominance.[10] Similarly, men with Klinefelter syndrome have two X and one Y chromosomes, resulting in low testosterone levels, increased gonadotropins, and high estrogen concentration; thus, patients with this disorder are immunologically similar to females, with higher immunoglobulin levels, increased CD4 + T-cells, CD4/CD8 T-cell ratios, and B-cells than XY male controls. These XXY cases have a 14-fold increase in the prevalence of SLE compared with general male population, thus equalizing the risk of the female population.[11]

To avoid double dosage of X chromosome-derived proteins, in women, one of the X chromosomes is randomly silenced during the early stages of embryogenesis, as suggested many years ago by Lyon.[12] Although one of the X chromosomes is almost completely silenced, approximately 15% of the genes escape X chromosome inactivation (XCI), leading to overexpression of some X-linked genes in female,[13] and among these genes, up to 10% are differently spared from XCI in different individuals;[14] thus, women are functional mosaics for X-linked genes.

XCI is generally random in all somatic tissues, and once chosen, it is stable with the same chromosome being inactivated in progeny cells; however, in some cases, the XCI can be skewed for stochastic reasons or due to sever mutations; skewing happens when one of the two alleles (either maternal or paternal) is in the active X chromosome in more than 75% of the cells.[15] It appears that skewed XCI is more prevalent in patients with autoimmune diseases than in healthy controls, in particular in female-preponderant disorders such as autoimmune thyroid diseases and SSc,[16] while in others, such as PBC, it has not been confirmed.[17] Although the exact mechanism is still unclear, skewed XCI might lead to a situation in which X-linked antigens are insufficiently expressed in immunological critical tissues such as the thymus but with a sufficient expression in peripheral compartments, thus creating the stimuli for the break of immune tolerance and explaining the production of autoreactive lymphocytes. Another hypothesis is that skewed XCI leads to the inactivation of a gene protective against autoimmunity or alternatively to the overexpression of a susceptibility gene leading to increase in autoimmune diseases' prevalence.[18]

A further mechanism studied as a genetic influence on autoimmune disease is the chromosome X monosomy, as suggested by the increased prevalence of autoimmune diseases in patients with Turner syndrome[19] and by the significant increase of X monosomy in peripheral blood cells of patients with PBC, autoimmune thyroid diseases, and SSc; a possible explanation of how this mechanism might be related with autoimmunity is that X chromosome monosomy may cause an haploinsufficiency in X-linked genes that escape XCI, so that autoreactive T-cells are not exposed to self-antigens encoded by one of the two X chromosomes and react against them generating an autoimmune response.[20]

Sex hormones

Due to the presence of hormones receptors on immune cells,[21] it appears that steroidal sex hormones may affect the strength and quality of immune responses in opposite directions: testosterone and progesterone seem to have an overall immunosuppressive effect and estrogen – in particular 17-β estradiol – and prolactin may have generally enhancing effects at least on humoral immunity.[4] Estrogens seem to switch the immune system toward the T-helper 2 (Th2) lymphocyte dominance with the consequence of more B-cell activation and antibody production.[22] In contrast, androgen favors the development of a Th1 response and CD8 + cell activation.[23]

These hormones have different effects depending either on their concentration or the targeted cells they exploit their activity on. For example, high concentrations of estradiol (E2), such as during pregnancy, exert mostly anti-inflammatory effects, by the inhibition of production and signaling of pro-inflammatory cytokines, such as tumor necrosis factor (TNF), interleukin (IL)-1 β and IL-6, and natural killer (NK) cell activation; by the induction of anti-inflammatory cytokines in favor of a Th2 phenotype, such as IL-4, IL-10, and transforming growth factor-β; and by the activation of T regulatory cells (Treg). Conversely, low concentration of E2 stimulate TNF, interferon-γ, IL-1 β, and NK cells, while enhancing antibody production by B-cells both at high and low concentrations.[24] These data point out that estrogens are capable of modulating both pro- and anti-inflammatory activities of CD4+ T-cells and thus have the potential to influence the outcome of CD4+ T-cells immune responsiveness. Prolactin increases the antibody production, regulates the development of CD4+ T-cells, and triggers the pro-inflammatory cytokine production;[25] progesterone stimulates a switch from a predominantly pro-inflammatory to an anti-inflammatory immune response, promoting skewing of CD4+ T-cell responses from Th1-type toward Th2-type responses, favors Treg differentiation, and exerts an inhibitor effect on NK cells.[26] Several studies indicate that testosterone and androgens have suppressive effects on the immune system by inhibiting the pro-inflammatory cytokine production, Th1 differentiation, immunoglobulin production, and NK cell cytotoxic activity and by potentiating the expression of anti-inflammatory cytokines[27][Figure 1]. Sex hormones modulate the hypothalamic–pituitary–adrenal axis and are capable to modulate the stress response. Indeed, women have higher corticosterone–cortisol concentrations compared to men, and glucocorticoids suppress the production of sex hormones and the production of these hormones in tissues.[28]

Along with the effects of sex hormones on the immune system and possibly disease susceptibility, severity changes of autoimmune diseases are observed during pregnancy, when estrogens and progesterone reach their highest peak.[29] Essential to maintain the maternofetal immune tolerance is the maternal adaptation leading to an immune shift from a pro-inflammatory Th1 response toward a Th2 response; consistent with this adaptive changes in immune system function, pregnancy has opposite effects on some autoimmune diseases, i.e., it is associated with an increase in disease flares in SLE, this effect being related to the increased Th2 response and enhanced production of pathogenic autoantibodies.[30] In contrast, pregnancy has a protective effect in Th1-dominant immune diseases, such as RA[31] and multiple sclerosis.[32]

Gut microbiota

Several animal studies have reported different effects of the gut microbiota on the mouse immune system; for example, normal development of lymphoid tissues in the gut is influenced by interactions with intestinal bacteria, and several immune cell populations show deficiency in their number and function in germ-free mice.[33] Based on this evidence, it is thought that the gut microbiota might play an important role in shaping the immune system even in humans. On the other hand, it is well established that immune system influences the composition of gut microbiota, generating a bidirectional interaction.[34] The effect of this bidirectional interaction supports the view that differences in gut microbial composition might explain the gender bias observed in autoimmune diseases; it has been demonstrated that the increased sensitivity to develop type 1 diabetes mellitus in female mice and the resistance of male mice to this disorder can be directly attributed to commensal microbiota;[35] the observation that male and female gut microbiota diverges after puberty implies that even sex hormones drive the composition of commensal microbes and this further exerts a protective or detrimental effect on susceptibility to autoimmune diseases.

More interestingly, the effect of the bidirectional interaction between the host and the microbes on disease can be transferred between animals, as protection from type 1 diabetes mellitus was lost in germ-free male mice, while protective effects of commensal bacteria could be transferred in early puberty from male mice to female mice.[35] This raises the hope that beneficial properties of microbiota may be used for microbiota transplantation and similar types of manipulation as therapies for autoimmune disease, even though our knowledge of such manipulation is still very limited.[36]

Why not Men?

As in the yin and yang, we may admit that autoimmune diseases show a lower prevalence in men than in women, with the exceptions of some male-predominant chronic inflammatory disorders, such as ankylosing spondylitis and primary sclerosing cholangitis,[23] and this may have complementary importance to what previously discussed. Compared to the X chromosome, the Y chromosome has received less attention due to the limited number of genes mapping on this chromosome;[27] however, research has given a twist to this assumption when the genetic mechanisms of BXSB strain of mice were discovered. These animals were found to spontaneously develop a syndrome similar to SLE but affecting male mice with a more severe phenotype than in females.[37] This manifestation was secondary to a genetic anomaly known as Y-linked autoimmune acceleration, i.e., a translocation from the telomeric end of the X chromosome (where the gene for TLR7 maps) onto the Y chromosome.[38] This evidence suggests that X-linked genes translocated on the Y chromosome may initiate and perpetuate autoimmunity. Excluded genetic abnormalities, the Y chromosome is thought to play a protective role in the setting of autoimmune diseases, but data are lacking.

Other than biological influences, is it possible that autoimmune diseases in men are underdiagnosed for reasons different from true prevalence of these disorders? It appears reasonable looking for the answer to this question in the fact that physician awareness plays a determinant role in assessing the probability of a possible autoimmune disease diagnosis, especially when the latter is subtle in nature.[7] Such standpoint is demonstrable by the opposite example of coronary heart disease – predominant in men – in which a video-simulated test was proposed.[39] This study demonstrated that women and men receive different care based on their sex; women are asked less questions and are ordered less diagnostic tests, resulting in less specialist referral, prompt diagnosis, and adequate treatment.

Is Sex a Determinant of Autoimmune Diseases Course?

To evaluate whether sex can predict the severity of autoimmune diseases, it is necessary to compare them in males and females and assess if their natural history changes during periods of sex hormone fluctuations (e.g., during pregnancy, before puberty, and after menopause). This evaluation has been made possible mainly through clinical observation and research studies on rodents.

Systemic lupus erythematosus

Much of the attention on sex bias in autoimmunity has been focused on SLE, which is often considered a “woman's disease” due to the higher incidence in females compared to males. The overall female-to-male prevalence ratio is around 9:1 in premenopausal age and declines to 4:1 in pediatric age and 5:1 in postmenopausal age-matched individuals; this evidence suggests that both sex hormones and reproductive activity might be important factors in the age of onset and sex bias in SLE.[40] Studies in experimental models show that lupus can be promoted by E2 via estrogen receptor α and mice deficient for this protein develop a milder disease, with decreased proteinuria and renal disease activity,[41] while in one study, progesterone appeared to have a protective effect in female mice, attenuating immune dysregulation and target organ damage.[42] Thus, it appears that estrogen and progesterone may induce different effects on autoantibody production, implying that in predisposed women, the balance between these two hormones could influence disease development and progression. According to this evidence, women who present a first episode of SLE tend to have lower levels of androgens and progesterone, while estrogen concentrations are normal;[43] although this hormonal imbalance theoretically explains the typical immune system alterations of SLE, i.e., autoantibody production by dysregulated B-cells, organ infiltration by inflammatory T-cells, and aberrant immune cell activation, whether this imbalance precedes disease onset in humans is not yet known.

On the other hand, SLE evolves with different onset, clinical features, and outcomes in men, suggesting the existence of male-specific predisposition and pathogenesis mechanism. There is little evidence for sex hormonal milieu to play a major role on male lupus,[44] and potential risk factors are identified in X chromosome abnormalities (as suggested by SLE susceptibility in patients with Klinefelter disease), other somatic genetic polymorphisms, and environmental factors, such as infectious agents, ultraviolet lights, and cigarette smoking.

Rheumatoid arthritis

Even though a female-to-male prevalence ratio of 3:1 may suggest an important role for sex hormones in the development of RA, evidence demonstrates that their influence is much less determinant compared to SLE; In fact, RA incidence peak in women corresponds to postmenopausal age – from 45 to 75-, thus putting factors other than estrogen and progesterone under the spotlight when talking about RA onset.[40] On the other hand, female sex hormones seem to influence disease activity in different and still unclear ways: higher disease activity scores were reported in women than in men[45] and significantly elevated estrogen levels in synovial fluid in both male and female RA patients[46] may indicate E2 as a detrimental factor on disease activity, but beneficial effects were obtained with the administration of higher doses of female sex hormones in animal models.[47] Thus, female sex hormones might require a pregnancy-like concentration to exploit antirheumatic effects. Conversely, androgens levels show an inverse association with disease onset and severity, and this evidence explains the decreased incidence of RA in men before 55 years and the decreased severe course of the disease as compared to age-matched women.[48]

The Impact of Pregnancy

Pregnancy is physiologically associated with a switch from a preponderant Th1 immune response toward a Th2 response; thus, it is logical to expect a remission of disease activity for AID characterized by Th1 immune activation, such as RA,[31] while disorders with a stronger Th2 composition, such as SLE and antiphospholipid syndrome, should experience disease worsening.[49] Despite these observations, the common clinical experience states that in the setting of rheumatic diseases, pregnancy represents one of the most delicate situations for disease remission or flares are not completely predictable and the spectrum of available medications narrows due to the risk of pregnancy adverse outcome. In this view, although data regarding most of therapeutic options' safety in pregnancy are currently lacking or inadequate, it appears important to understand whether the efficacy of antirheumatic drugs varies between pregnant and nonpregnant population.

Accordingly with this assumption, when a young woman is diagnosed with AID, the rheumatologist is entrusted with multiple responsibilities: optimizing disease control before conception, effective and safe utilisation of DMARDs throughout pregnancy require open dialogue and shared decisions.[50] The Food and Drug Administration provides recommendations for the use of any medication during pregnancy [Table 2]. Aiming to optimal patient management, care providers must be able to counsel the patient regarding the risks and benefits of these drugs and individualize therapeutic regimens for each patient to optimize the chances for a healthy pregnancy and healthy newborn. This can be achieved by choosing the most suitable DMARDs in accordance with the patients' desire for pregnancy, education regarding appropriate contraception, while timing the pregnancy when AID is stable and ensuring adequate follow-up during pregnancy.

Table 2: Summary of recommendations for safety of principle therapies for AID during pregnancy

During the past two decades, discoveries about immune system architecture and function have been exponentially increasing and led to a deeper comprehension of immunological mechanisms and background behind autoimmune diseases. The basis for personalized medicine is that unique aspects of our biology, including our immune response, will define novel targets for more effective prevention and treatment of immune-related diseases.[51] Since sexually dimorphic prevalence of autoimmune diseases remains one of the most intriguing clinical observations among this group of diseases, a personalized approach to autoimmunity must take into account of gender medicine.

Recent intuitions on sex difference in immune responses highlighted the interest around sex bias in these disorders. Thus, sex chromosomes, sex hormones, and their interaction with element extrinsic to the host, such as the gut microbiota, become more defined actors of the susceptibility, pathogenesis, and severity of autoimmunity. While autoimmune diseases are considered multifactorial, thinking that gender may represent the only causal agent is certainly simplistic, however to date it is clear that genetic and hormonal sex-related features influence not only the onset and course of the diseases but also the outcomes to therapies.[52],[53] These data encourage further investigation on sex mechanisms influencing autoimmune diseases, so that we might eventually exploit sexual dimorphism for a more accurate management of individual patients with autoimmune diseases, employ sex hormones as therapies adjuvants, and intercept patients most in need for such therapeutic strategies.